1. Field of the Invention
The present invention relates to methods of cutting thin film filter work pieces for optical devices, and more particularly to methods of cutting dense wavelength division multiplexing (DWDM) thin film filter work pieces that reduce their residual stresses and thus reduce their stress-induced peak insertion losses.
2. Description of Prior Art
Thin films are an applications branch of optical and semiconductor science and technology that has had a tremendous impact on social development, especially in the electronic information and optical communications fields. These films can be single layer or multiple layer coated on a substrate. They can also be freestanding components in their own right.
Peak insertion loss is an important criterion for evaluating the quality of a thin film filter. Today more precisely controlled techniques are being developed, which enable more layers of film to be deposited on a given substrate. This enables DWDM thin film filters to meet increasing demands for increased number of cavities and smaller distances between channels. Such techniques include chemical vapor deposition (CVD), ion beam aided deposition (IBAD), Sol-Gel, and vacuum evaporation. Unfortunately, the increased number of film layers necessarily results in more internal stress in a film stack of a DWDM thin film filter, which results in increased peak insertion loss. If the film stack of a DWDM thin film filter work piece already has high internal stresses after formation, it is important that these stresses not be unduly increased as a result of cutting. Indeed, such a film stack is often too brittle to be safely cut. T he film stack may be damaged or destroyed during cutting.
The most common method of cutting a DWDM thin film filter work piece is to prepare the work piece so that it comprises a film stack on a glass substrate. The work piece is mounted on a piece of ultraviolet (UV) tape, and the resulting combination tape-work piece is mounted on a dicing machine. The combination tape-work piece is then cut with a diamond blade by the dicing machine to form pieces of desired size.
U.S. Pat. No. 6,295,978 discloses a conventional method for dicing wafers, which could be applied to thin film filter work pieces. Referring to FIGS. 7 and 8, a wafer has a circuit side, an underside and a street index that defines a guide for dicing the wafer into pieces. The method includes providing a chuck having a surface for supporting the underside of the wafer. A plurality of recesses is defined in the upper surface of the chuck, corresponding to the street index of the wafer. When a cutting blade cuts along a street index, the recess beneath the street index saves the blade from unnecessary wear.
The methods mentioned above are well-established for cutting wafers or ordinary thin film filter work pieces having relatively few layers of film. However, modern DWDM thin film filters can have more than 150 layers of film. When the blade of the dicing machine is cutting the work piece, additional stress is induced in the film stack, and this stress is added to the pre-existing internal stresses within the film stack. The stress induced by the blade results in increased residual stress in the film stack. This increases the peak insertion loss of the final product thin film filter, or even distorts the whole optical spectrum of the final product thin film filter. The stress induced by the blade may even destroy the film stack during cutting.
In view of the above, an object of the present invention is to provide a method for reducing residual stresses in film stacks of diced thin film filter work pieces.
A further object of the present invention is to provide a method for increasing the effective thickness of a thin film filter work piece, thereby increasing the allowable yield stress in the work piece, minimizing addition of cutting-induced stress to residual internal stress in the film stack of diced work pieces, and thereby reducing stress-induced peak insertion loss accountable to cutting of the work piece.
Another object of the present invention is to provide a method for reducing the risk of a film stack peeling from a substrate during cutting of a thin film filter work piece.
To achieve the above objects, a method of cutting thin film filter work pieces in accordance with the present invention is described in two preferred embodiments. In a first embodiment, an intermediate layer is attached between an augmenting substrate and a glass substrate of the work piece prior to cutting of the work piece. In a second embodiment, the work piece comprises a glass substrate which is thicker than a final desired thickness. After the work piece is cut, a surplus part of the glass substrate is removed. Both of the embodiments of the present invented method increase the effective thickness of the work piece, which results in a reduced residual stress in the final product thin film filter, and which reduces the risk of a film stack of the work piece peeling from the glass substrate during the machining process.